Group-III nitride-based semiconductors are direct-transition type semiconductors exhibiting a wide range of emission spectra from UV to red light when used in a device such as a light-emitting device, and have been used in light-emitting devices such as light-emitting diodes (LEDs) and laser diodes (LDs).
When a light-emitting device has higher external quantum efficiency (the number of photons extracted to the outside/the number of injected carriers), the less power consumption can be achieved. The external quantum efficiency can be raised by increasing the light extraction efficiency (the number of photons extracted to the outside/the number of emitted photons) or the internal quantum efficiency (the number of emitted photons/the number of injected carriers). The increase of the internal quantum efficiency means the decrease of the energy of the heat converted from the electricity given to the light-emitting element. Therefore, it is considered that the increase of the internal quantum efficiency not only reduces the power consumption but also suppresses the lowering of the reliability due to the heating.
The extraction efficiency of an LED can be much improved by either growing or mechanically bonding the lower confining layer upon a transparent substrate rather than an absorbing one. The extraction efficiency of a transparent substrate LED is reduced by the presence of any layers in the LED that have an energy gap equal to or smaller than that of the light-emitting layers. This is because some of the light that is emitted by the active layer passes through the absorbing layers before it exits the LED. These absorbing layers are included because they reduce the number of threading dislocations or other defects in the active layer or are used to simplify the LED manufacturing process. Another effect is to reduce band offsets at hetero-interfaces, which lower the voltage that must be applied to the contacts in order to force a particular current through the diode. Because the absorbing layers tend to absorb shorter-wavelength light more effectively than longer-wavelength light, LEDs that emit at 590 nm suffer a greater performance penalty due to the presence of these layers than LEDs that emit at 640 nm.
Absorption in the active region also reduces the extraction efficiency. In the prior art, techniques for improving the efficiency of LEDs have focused on determining the active layer thickness which results in greatest internal quantum efficiency and on increasing the extraction efficiency of the LED by removing the absorbing substrate. The extraction efficiency can be further improved by making all absorbing layers, including the active layer, as thin as possible. However, ultra-thin active layers may result in a decrease in the internal quantum efficiency of the LED.
As mentioned above, absorbing layers are included because they reduce the number of threading dislocations, and therefore, an alternative method which can also reduce the number of threading dislocations may overcome the aforementioned problem caused by the absorbing layers.
When a group-III nitride-based semiconductor is formed on a silicon (Si) substrate, epitaxial growth process may be carried out under condition that stress owing to misfit of lattice constants between the silicon (Si) substrate and the group-III nitride-based semiconductor is always applied. Difference of thermal expansion coefficients between the silicon (Si) substrate and the group-III nitride-based semiconductor increases the stress in a cooling process, to thereby generate a lot of cracks (fractures) in the group-III nitride-based semiconductor layer. As a result, cracks generated in a region where a light-emitting device or other device is formed make the device a defective product and because of that yield rate of the device becomes remarkably poor.
When a selected growth process is carried out so as not to generate cracks, actually stress cannot be relaxed sufficiently and especially threading dislocations do not decrease. In short, generation of cracks means relaxation of stress owing to the cracks. So when the cracks are suppressed, stress is always applied to threading dislocations, and therefore, preventing upward propagation of each threading dislocation is desperately desired.
The present invention has been accomplished in an attempt to solve the aforementioned problems, and an object of the present invention is to form an oxide layer such as a ZnO layer in replace of an absorbing layer to prevent threading dislocation from propagating upwards, which may further enhance internal quantum efficiency and improve light extraction efficiency of the group-III nitride-based semiconductor.